This invention relates to video noise reducers and method's of noise reduction.
A well known method for the reduction of noise In a video signal employs a recursive Filter, The method operates by averaging similar areas in successive images and relies on the fact that the true image information in a video signal is highly correlated, Noise, however, is generally random and is attenuated in the averaging process, A recursion constant k determines the relative weightings of the current and preceding frames in the averaging process and thus the number of frames over which effective averaging takes place. Significant attenuation of noise can be achieved with averaging over a relatively small number of frames.
It is well understood that the picture correlation upon which recursive noise reduction relies, breaks down where there is movement in the input images. It is important to disable the recursion when movement is detected; failure to do this will result in image smear. Motion adaptive noise reduction has the annoying artifact, that picture regions at the edges of moving objects are substantially noisier than the surrounding picture. An edge of a moving object is associated with disabling of recursive noise reduction so that each edge of the moving object will be followed by a region of signal which is not noise reduced. The problem is exacerbated by the need--usually--for the motion detector signal to be filtered spatially; this results in an even larger area which lacks noise reduction. It is also the case that the more effective is the noise reduction process over the image as a whole, the more noticeable are the noisy regions around moving objects.
Whilst the need to avoid smearing has been described in relation to the detection of moving objects, the disabling of recursion will apply also to global motion such as panning and also to shot changes and other edits.